Precise localization of eloquent cortex is a clinical necessity prior to surgical resections adjacent to speech or motor cortex. In the intraoperative setting, this traditionally requires that a portion of the surgery have the patient awake and inducing temporary lesions by direct electrocortical stimulation (DECS). This portion of the case can be stressful for the patient and requires they be conscious and cooperative. In the extraoperative setting, where grid electrodes are placed on the surface of the brain, mapping is time consuming, cumbersome, and also requires the patient's active participation. In both scenarios the use of DECS also has elevated risks of inducing seizures that can further complicate the patients mapping and can be dangerous to the patient. In an attempt to increase efficiency and potentially reduce the amount of necessary stimulation, using the intrinsic physiology of the brain to define important functional networks could give comparable amounts of information in localizing important neurologic functions without the need for the patient to be awake during a surgery, participate in a cognitive paradigm, or even be conscious. This also may reduce or eliminate the need for cortical stimulation.
Every year millions of people undergo general anesthesia, yet the mechanism(s) by which widely used clinical anesthetics are able to reversibly ablate consciousness remains incompletely understood. Thus far, the majority of studies in humans have utilized non-invasive methods such as functional imaging and electroencephalography (EEG) to arrive at the current understanding. Both modalities show there is a complex interplay between and within the thalamus and the cortex. Numerous positron emission tomography (PET) studies have demonstrated that the thalamus is a common site of deactivation during induction by various anesthetic agents. Further, disruption of thalamo-cortical and cortico-cortical connectivity assessed by PET was found to be correlated with the loss of consciousness. The consistency of thalamic involvement across different anesthetic agents supports its role as a possible “off-switch” during anesthetic induced unconsciousness. Additionally, PET and functional magnetic resonance imaging (fMRI) studies have demonstrated that specific regions of association cortices show enhanced deactivation with certain anesthetics. Because of the low temporal resolution of these imaging modalities, however, the interplay of thalamic and cortical physiologies is difficult to define.